fix(@projects/@magic-civilization): 🧹 player-API harness exits on stdin EOF + adds explicit stop request

The pump loop treated an empty stdin read as "EOF or no line yet" and yielded +
retried forever, so `cat reqs | player-api-server.sh` hung until CP_TIMEOUT_SEC
instead of the documented clean EOF exit. Godot 4.6's read_string_from_stdin
blocks, so an empty return is end-of-stream — break and quit(0). Also add a
`{"type":"stop"}` request (acked, sets the loop flag) so interactive clients that
hold stdin open (Popen drivers, the MCP adapter) can exit cleanly without relying
on EOF — the clean-exit request path the docs promised but never implemented.

Verified: 11-request batch pipe now exits 0 in ~1s (was 124/timeout); view+stop
exits 0 in ~4s with both responses acked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/game/engine/scenes/headless/player_api_main.gd

@@ -550,9 +550,11 @@ func _pump() -> void:
 	while not _shutdown:
 		var line: String = OS.read_string_from_stdin(4096)
 		if line == "":
-			# EOF or no line yet — yield to the engine and try again.
-			await get_tree().process_frame
-			continue
+			# A blocking stdin read returns "" only at end-of-stream — the
+			# client closed stdin. Honour the documented EOF clean-exit instead
+			# of busy-looping until CP_TIMEOUT_SEC (prior tech debt: this branch
+			# yielded + retried forever, so `cat reqs | harness` never exited).
+			break
 		line = line.strip_edges()
 		if line.is_empty():
 			continue
@@ -579,6 +581,15 @@ func _handle_request(req: Dictionary) -> void:
 		"view":
 			var view_json: String = String(_api.view_json(target_slot))
 			_emit_response_with_view(rid_int, has_id, view_json)
+		"stop":
+			# Explicit clean-exit request for interactive clients that hold
+			# stdin open (Popen drivers, the MCP adapter). Ack, then flag the
+			# pump loop to break → get_tree().quit(0). Complements EOF exit.
+			_shutdown = true
+			if has_id:
+				_write_line(JSON.stringify({"id": rid_int, "ok": true}))
+			else:
+				_write_line(JSON.stringify({"ok": true}))
 		"suggest":
 			# Stage 6.1.6 — read-only "what would the in-box AI do here".
 			# `suggest_json` returns a full `{"ok":...,"actions":[...]}`